This application claims the priority of German application 102 05 380.4, filed Feb. 9, 2002, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a method and a device for removing particles, which have been deposited on a filter arranged in an exhaust system of a diesel internal combustion engine, by combustion.
To reduce the level of harmful exhaust gases from diesel internal combustion engines, and in particular to reduce the levels of soot particles, filters, in particular soot filters, are used in the exhaust system. These filters have the problem that they rapidly become blocked by the large quantities of soot which is produced and thereby become ineffective. For this reason, it is desirable for the soot on such filters to be eliminated by oxidation or combustion.
It is known from European publication EP 341 832 to form NO2 in the exhaust gas from NO-containing exhaust gas by means of a platinum catalytic converter. This NO2 makes it possible to burn soot at a temperature of less than 400xc2x0 C. i.e. at the normal exhaust gas temperature. In this case, the problem exists that the diesel fuel has to have a very low sulphur content. Otherwise, the catalytic converter is poisoned and the soot in the filter can no longer be burnt as intended. However, sulphur-containing fuel is not available everywhere; this is a problem which, in particular, affects lorries used internationally.
It is known from U.S. Pat. No. 5,522,905 to add certain additives to the fuel of a diesel internal combustion engine. This reduces the ignition temperature of the soot in the filter to such an extent that combustion is possible at standard exhaust gas temperatures. These additives contain, inter alia, metal salts of an organic compound. In this context, the problem arises that the additives are expensive.
One object of the present invention is the object of providing a method and also a device which allow soot to be eliminated from a filter of a diesel internal combustion engine in an economically and ecologically appropriate way under a wide variety of conditions. In particular, the method and the device are also to be protected against incorrect intervention.
This object is achieved by passing NO-containing exhaust gas over a catalytic converter upstream of the filter in order to convert the NO in the exhaust gas into NO2, and then using NO2-containing exhaust gas to burn the particles which have been deposited on the filter when the sulphur content is lower than a predetermined sulphur content, the quantity of NO which has been converted into NO2 being sufficient to enable the combustion of the particles to be carried out at a temperature of less than 400xc2x0 C., and adding an additive which contains a metal salt of an organic compound to the fuel in order to reduce the ignition temperature of the particles which have been deposited on the filter to below the normal ignition temperature of the particles when the sulphur content is higher than the predetermined sulphur content. The object is also achieved by a device including an exhaust pipe having a filter and a catalytic converter, which is arranged upstream of the filter, for converting NO in the exhaust gas into NO2, additive-adding means for adding additive to the fuel, and sulphur content-measuring means for determining a sulphur content in the fuel. Advantageous and preferred configurations of the invention form the subject-matter of further claims and are explained in more detail below. The content of the claims is made part of the content of the application by express reference.
A method according to the invention has two possible method steps, a) or b). In a first method step a), NO-containing exhaust gas is passed over a catalytic converter, for example an oxidation catalytic converter, upstream of the filter. NO in the exhaust gas is converted into NO2, which is then used to burn .the particles which have been deposited on the filter. A quantity of NO2 which is sufficient to carry out the combustion of the particles at a temperature of less than 400xc2x0 C., in particular at standard exhaust gas temperature, is formed. For details in this connection, reference is expressly made to European publication EP 341 832, the content of which is hereby incorporated in the present description.
According to another method step b), adding an additive to the fuel is possible. This additive contains a metal salt of an organic compound, with the result that the ignition temperature of the particles which have been deposited on the filter is reduced to below the normal ignition temperature of the particles, in particular to the normal exhaust gas temperature.
Furthermore, according to the invention the sulphur content in the fuel is determined in order to carry out one method step or the other as a function of this measurement. If the sulphur content is lower than a predetermined sulphur content, method step a), i.e. the generation of NO2, is carried out. On the other hand, if the sulphur content is higher than a predetermined sulphur content, method step b), i.e. the addition of additive, is carried out. Although in principle NO2 may be generated as well, because the catalytic converter has generally been deactivated by the high sulphur content, scarcely any NO2 is actually generated.
This has the advantage that the poisoning of the catalytic converter has no adverse effect on the deposits of soot on the filter, but rather the soot or particles is/are still burnt. If the sulphur content, i.e. the content of the substance which poisons or deactivates the catalytic converter, is measured in order to make this decision, it is possible to obtain a precise indication of the ability of the catalytic converter to function and the possibility of carrying out method step a).
Advantageously, in each case only one of the two method steps is carried out. In principle, method step a), i.e. the generation of NO2, is particularly advantageously carried out. Method step b) is only carried out when the predetermined sulphur content has been exceeded.
It is possible and even, with a view to simplify the exhaust-gas management, advantageous for the exhaust-gas stream to be passed over the catalytic converter in accordance with method step a) even when method step b) is being carried out. However, since method step b) is only used if the sulphur content is too high, this sulphur content deactivates the catalytic converter and method step a) ceases to work.
There are various options for determining the sulphur content in the fuel. Firstly, it is possible to measure the exhaust gas back pressure upstream of the filter. If the sulphur content is too high, the catalytic converter is poisoned and its NO2 formation function is reduced or eliminated. This leads to a rise in the exhaust gas back pressure, since the soot-burning operation effected by NO2 is disrupted and the filter becomes clogged. For example, the fact that a certain exhaust gas back pressure has been exceeded can be deemed an indication of a predetermined sulphur content having been exceeded. The level of the sulphur content can be at least approximately determined from the extent to which the exhaust gas back pressure has been exceeded.
According to an alternative or additional option, it is possible to determine the sulphur content in the fuel, in particular in the fuel supply to the internal combustion engine. This can be achieved, for example, using a sulphur sensor. On the one hand, this may take place in a tank which holds the fuel. On the other hand, a determination of this type may also be performed in a fuel feed line to the internal combustion engine itself. A measurement of this type advantageously takes place independently of fuel mixing during a plurality of different refueling stops.
It is possible for the sulphur content to be determined in each case at predetermined time intervals. This makes it possible to reduce costs. The sulphur content should be predetermined at least each time the vehicle is refuelled. The sulphur content in the predetermined fuel can only change as a result of the type of refuelling. In this context, it conceivable to detect a refuelling operation by means of a suitable sensor on a tank cover, on the level-measuring means or the like and to trigger a measurement of the sulphur content.
Alternatively, it is possible to determine the sulphur content in the fuel on an ongoing basis. This allows accurate determination of the actual state. Determination of this type may take place in a fuel tank or a fuel feed line.
According to one possible embodiment of the invention, the addition of additive in accordance with method step b) may be metered. The metering preferably takes place as a function of the predetermined sulphur content being exceeded. This makes it possible to avoid either too little or too much additive being used.
Once again, as with the measurements of the sulphur content, there are a number of possible options with regard to the addition of the additive. In principle, it is possible for additive to be added to the fuel all in one go into the tank each time the vehicle is refuelled. The quantity required for this purpose can be determined from the sulphur content and the tank volume or refuelling delivery.
According to a further possibility, the additive may also be added to the fuel discontinuously, i.e. a number of times each time the vehicle is refuelled. By way of example, this allows discontinuous combustion, at intervals, of particles which have been deposited on the filter. In this way, additive is not added permanently.
Finally, it is also possible for the additive to be added to the fuel substantially continuously. This advantageously takes place into a fuel line to the internal combustion engine. In this case, it is possible in each case to measure the sulphur content on an ongoing basis and to meter the additive accordingly.
In addition to a metal salt of an organic compound, the additive may also include an alkaline metal salt from the group of the rare earths. It is particularly advantageous for these two metal compounds to be the only metal compounds in the additive. For further details, in particular possible compositions of the additive, reference is made to U.S. Pat. No. 5,522,905 referred to above, the content of which is hereby incorporated in the description.
The device according to the invention is distinguished by the fact that it has an exhaust pipe with a filter and a catalytic converter arranged upstream of the filter. The catalytic converter is designed to generate NO2 from NO in the exhaust gas. There are additive-adding means for adding an additive to the fuel. Furthermore, there are sulphur content-measuring means for determining the sulphur content in the fuel. This device can particularly advantageously be used to carry out the method described above. The increased outlay compared to conventional devices remains within acceptable limits.
The catalytic converter provided may be a platinum catalytic converter, in particular with a honeycomb monolith. The additive-adding means may be connected to a tank or advantageously to a fuel supply or a fuel line to the internal combustion engine. They may include a stock of additive, for example in a storage container. Furthermore, there is a valve facility, by means of which the metering is effected. With regard to the structure, reference is made to European publication EP 341 832 mentioned above.
The sulphur content-measuring means may firstly have a pressure-measuring facility, for example gas pressure sensors. These may be arranged in the exhaust system upstream of the filter and record the rise in the exhaust gas back pressure above a defined, predetermined level.
As an alternative or in addition, the sulphur content-measuring means may have a sulphur sensor. The latter should be arranged in the fuel supply upstream of the internal combustion engine, either in a fuel line or in a fuel tank.
Furthermore, there is advantageously a control unit which causes additive to be added as a function of the state which has been determined by the sulphur content-measuring means or the sulphur content.
If the transmission from method step a) as the method step which is used in principle to method step b) in an exceptional situation takes place substantially automatically, combustion of the particles in the filter is ensured at any time. This is true both in situations in which no sulphur-containing fuel is available and in situations in which refuelling has been carried out with fuel with a sulphur content which is too high, either by mistake or because of the absence of accurate knowledge. Therefore, the way in which the combustion of the particles takes place is not dependent on an operator.